(i) Field of the Invention
The present invention relates to a method of producing ferrite single crystals. More particularly, the present invention relates to a process for producing ferrite single crystals in an oxygen atmosphere under an oxygen partial pressure higher than that in the air.
(ii) Brief Description of the Prior Art
Ferrite single crystals such as Mn-Zn ferrite, Ni-Zn ferrite, Mn ferrite, Ni ferrite, Co ferrite and Ba ferrite single crystals have been produced mainly by Bridgman's method or flux method in the air.
However, in those methods, ferrite is heated to a high temperature of about 1300.degree. C. in the crystallization step and, therefore, it is thermally dissociated to release oxygen and also to produce FeO, i.e. Fe.sup.2+, in a large amount. For example, it has been known that if Mn-Zn ferrite having a chemical formula: EQU Mn.sub.0.5 Zn.sub.0.5 Fe.sub.2 O.sub.4
is molten in the air, FeO formed amounts to 19 wt.%. It has also been known that if the above mentioned Mn-Zn ferrite is molten under an oxygen partial pressure higher than that in the air FeO formed is reduced in amounts as oxygen partial pressure is elevated and that FeO content of said Mn-Zn ferrite is about 2 wt.% when it is molten under an oxygen pressure of 5 atms. FIG. 1 is a graph showing a relationship between FeO content of Mn-Zn ferrite sample which has been kept at 1650.degree. C. for one hour and then cooled and oxygen partial pressure in the atmosphere in which the melting is effected, which graph is given in Mitsuo Sugimoto; "Proceedings of the 5th Symposium organized by Denkitsushin Kenkyu-sho of the Tohoku University, Crystal Growth, Dec. 1967, III-1".
It is desirable to elevate oxygen pressure for controlling amount of Fe.sup.2+ formed, since the presence of Fe.sup.2+ exerts a great influence on magnetic properties of the ferrite.
On the other hand, ZnO-containing ferrite single crystals prepared in the air, particularly, Mn-Zn ferrite and Ni-Zn ferrite single crystals, have significantly reduced Zn concentrations as compared with that of the starting material, since ZnO having a high vapor pressure is easily evaporated out at a high temperature.
It has also been known that when Mn-Zn ferrite single crystals having the chemical formula: EQU Mn.sub.0.6 Zn.sub.0.4 Fe.sub.2 O.sub.4
are grown by Bridgman's method, about 5 wt.% of Zn is evaporated out if it is molten in the air but the evaporation thereof is reduced in amount by elevating oxygen pressure. For example, if oxygen pressure is 5 atms., amount of Zn evaporated is less than 1 wt.% (see said literature).
Thus, if the ferrite single crystals are produced under an elevated oxygen pressure, amounts of FeO produced and Zn evaporated are reduced to obtain favorable results. The ferrite single crystals thus obtained are suitable for the production of a device having excellent magnetic properties.
However, when single crystals are produced in oxygen atmosphere by conventional methods, the gas in the melt pops out (hereinafter this phenomenon will be referred to as popping) to make it impossible to maintain the melt stably and, in addition, this is very dangerous, since the melt is scattered out in drops as the gas pops. Therefore, it has been difficult to obtain ferrite single crystals of high qualities by Bridgman's method, though the production of the single crystals in oxygen atmosphere is barely possible by this method. Still less, the production of ferrite single crystals by floating zone method in oxygen atmosphere has substantially been impossible.
The following references are cited to show the state of the art; (i) A. Ferretti, et al.; Journal of Applied Physics, Vol. 34, No. 2 (February, 1963), pp, 388-389, (ii) A. Ferretti, et al.; Journal of Applied Physics, Vol. 32, No. 5 (May, 1961), p. 905, and (iii) M. Sugimoto; Japan. Journal of Applied Physics, Vol. 5 (1966), pp. 557-558.